LESSON IV. 



CULTURE MEDIA. 



A variety of nutrient substances are in more or less general use in the culture of 

 bacteria, and such variety is essential, not merely for the ordinary requirement of 

 growing the organisms, but also for the further purpose of observation of the varying 

 characteristics presented by this or that bacterium upon the different types of media 

 and furnishing data useful for its identification. Of the different media suggested, 

 the most important for routine use are potatoes, bouillon, gelatinized bouillon ("gelatine"}, 

 agarized bouillon ("agar"), peptone solution, glucose-, lactose-, and saccharose-bouillon, 

 blood-serum, milk, and litmus-milk. In the preparation of these media it is important, 

 inasmuch as from variation of their constitution there may follow important alterations 

 of the characteristics of the bacteria grown upon them, that uniformity of manufacture 

 should be sought in order that the records of different individuals engaged upon the 

 same microorganisms may be uniform and comparable. It is desirable, too, that all 

 results observed as to the growth of any bacterium upon the standard or upon special 

 modifications of the common media should be recorded with clear indication of the exact 

 composition of the nutrient medium employed, as well as the other conditions of growth 

 prevailing. The reaction of the medium is an especially important feature; in which 

 matter it is to be recommended that the standard reaction adopted by the Bacterio- 

 logic Committee of the American Public Health Association be followed. It is to 

 be understood, except as may be indicated in the following instructions, that the reaction 

 of all media described should be adjusted to this standard: that to phenolphthalein (a 

 more delicate reaction indicator than litmus) the reaction of the medium will be -(-1.5 

 (acid 1.5 per cent. that is to say, each one hundred cubic centimeters of the medium 

 contains a sufficient excess of acid elements to require for their neutralization 1.5 cubic 

 centimeters of a normal sodium hydroxide solution). Although this is accepted as a 

 standard reaction, it will not infrequently be observed that variations presenting 

 greater or less percentage of acid, or which are actually alkaline, will be more favorable 

 for the growth of individual forms of bacteria ; and a certain number of such modified 

 media may be advantageously kept in stock for special work. The reaction is to be 

 indicated by the percentage quantity of acid or alkali (hydrochloric acid or sodium 

 hydrate) in normal solution required for the neutralization of one hundred cubic centi- 

 meters of the medium, the mark -f indicating acid, the mark , alkaline reaction. 

 Thus, bouillon, sufficiently acid to require for the neutralization of one hundred cubic 

 centimeters 0.5 cubic centimeter of a normal solution of sodium hydroxide, would be 

 indicated as -f 0.5 ; a bouillon of alkaline reaction sufficient to require for the neutraliza- 

 tion of one hundred cubic centimeters 0.75 cubic centimeter of a normal solution of 

 muriatic acid would be represented as 0.75 reaction. For the determination and ad- 

 justment of reaction there will be required the following solutions: (a) a normal solu- 

 tion of sodium hydroxide in distilled water (40 grams NaOH, 1000 cubic centimeters 

 water) ; (6) a decinormal solution of sodium hydroxide (4 grams to 1000 cubic centimeters 



70 



